G. W. Dalke and M. D. Buchanan in their U.S. Pat. No. 4,183,046 issued Jan. 8, 1980 and entitled "ELECTRONIC APPARATUS FOR CONVERTING IMAGE OR GRAPHICS DATA TO COLOR VIDEO DISPLAY FORMATS AND METHOD THEREOF" describe a digital video color generation system for converting image or graphics data into a color video display format. In their system, image or graphics data are transformed into a succession of data words. Each data word comprises three separate groups of data bits which define the video display to be produced in terms of intensity, hue and color saturation characteristics. Such data bit groups are stored in respectively corresponding fields in a memory to allow independent user control of one or more of such color characteristics. The data bit groups corresponding to hue and saturation are employed for simultaneously addressing storage locations in three color map memories which contain digital color reference data representing symmetrically mapped combinations of hue and saturation values. The data in these storage locations are read out as digital inputs to respective multiplying digital-to-analog converters for producing red, blue and green color signals. The group of data bits corresponding to the luminance of a picture element (or "pixel") are directly converted to an analog luminance, or Y, signal which controls the reference voltage input of each of the multiplying digital-to-analog converters. This is done in a manner which modulates the magnitude of each of the video signals, to permit varying the display luminance without also altering either the hue or color saturation.
Dalke and Buchanan describe a display processor in which three color map memories are used for storing data comprising red (R), green (G) and blue (B) color signals each normalized respective to the luminance signal. This data is recovered and multiplied by the luminance signal to generate red, green and blue color video signals. The normalization of R, G and B color signals with regard to luminance reduces the number of values of them that have to be stored in the color map memories in order to describe color space or a portion thereof with desired color resolution at all luminance levels. So, the number of bit places in the color map memory addresses can be reduced accordingly. This reduces the total size of the color map memories. Dalke and Buchanan, then, describe a "multiplicative" system of describing color space in terms of three "multiplicand" color signals (normalized R, normalized G and normalized B) and a single "multiplier" luminance signal (Y), which system requires three color map memories.
In broadcast color television systems which employ analog signals an "additive" system of describing color space is used. Color space is described in terms of a luminance signal (Y) and two orthogonal color-difference signals. A color difference signal is derived by subtracting the luminance signal Y from a particular color signal. This particular color signal can describe the intensity of an additive primary color such as R or B, as is the case with R-Y and B-Y color difference signals. Alternatively this particular color signal may describe the intensity of a complex color formed by a linear combination of additive primary colors, as is the case with the I and Q color-difference signals employed in NTSC broadcast color television. For a more complete background concerning the subject of color differences the reader is referred to D. L. MacAdam's book COLOR MEASUREMENT; THEME AND VARIATIONS published in 1981 by Springer-Verlag, Berlin, Heidelberg, N.Y., especially chapter 8 entitled "Color Differences". (R-Y) and (B-Y) color-difference signals are used in European broadcast color television, and a pair of color-difference signals I (along a cyan-orange axis in the CIE chromaticity diagram) and Q (along a green-magenta axis in the CIE chromaticity diagram) are used in North American and Japanese broadcast color television. "Additive" systems describing color space are favored in broadcast practice because they allow one to conserve signal bandwidth. Relying on the fact that acuity for chrominance variation is less acute than for luminance variation in the human visual system, chrominance is transmitted with less spatial resolution than luminance.
A "multiplicative/additive" system of describing color space is employed when color mapping in accordance with the present invention. Color or color-difference signals normalized respective to luminance are supplied from respective map memories as multiplicands, which are multiplied by a luminance signal multiplier in arithmetic processes that generate color-difference signals free of normalization respective to luminance. This multiplicative procedure is followed by the linear combination of these color-difference signals with the luminance signal in color matrixing circuitry, for generating color signals to drive a color display apparatus. Certain multiplicative/additive systems can reduce the number of map memories needed to generate a polychromatic display from three to two, saving memory over the Dalke and Buchanan color mapping scheme. The multiplicative/additive system also permits the map memories used to generate such a polychromatic display to be operated using a narrower spatial bandwidth than is otherwise used for luminance without introduction of objectionable aliasing terms. This is not possible with a multiplicative system of describing color space as used by Dalke and Buchanan.
Normalization of chrominance respective to luminance obviates problems of luminance/chrominance tracking. The descriptions of luminosity and chromaticity become truly separable. This facilitates non-linear coding of luminance being carried ou independently of chrominance processing, non-linear coding of chrominance being carried out independently of luminance processing, or both. A digital representation of chrominance which is not normalized respective to luminance tends to have excessive quantizing error at lower luminance levels. Digital chrominance normalized respective to luminance has the same relative quantizing error at all luminance levels. This provides a better approximation to the chromaticity resolution requirements of human vision. The constant relative quantizing error in chrominance at all luminance levels facilitates the non-linear processing of luminance without the risk of attendant dilution of chrominance resolution.